People living in the margins. Youths smarting from brushes with the law. Religious converts who, despite their fervor, lack a deep understanding of bedrock teachings.

This is the profile of people drawn to extremist Islamic groups, scholars said.

And while there are still many question marks about 33-year-old Douglas Authur McCain — who last weekend became the first American to die while fighting for the jihadist group Islamic State — some of this profile may be a match.

Friends called him “Toothless.”

This March 23, 2008 photo provided by the Hennepin County, Minn. Sheriff's Office shows Douglas McAuthur McCain. The Obama administration has offered a wide range of assessments of the threat to U.S. national security posed by Islamic State extremists in an area straddling eastern Syrian and northern and western Iraq, and whose actions include last week’s beheading of American journalist James Foley. Some officials say the group is more dangerous than al-Qaida. Yet intelligence assessments say it currently couldn’t pull off a complex, 9-11-style attack on the U.S. or Europe. (AP Photo/Hennepin County, Minn. Sheriff's Office)The Associated Press

This March 23, 2008 photo provided by the Hennepin County, Minn. Sheriff's Office shows Douglas McAuthur McCain. The Obama administration has offered a wide range of assessments of the threat to U.S. national security posed by Islamic State extremists in an area straddling eastern Syrian and northern and western Iraq, and whose actions include last week’s beheading of American journalist James Foley. Some officials say the group is more dangerous than al-Qaida. Yet intelligence assessments say it currently couldn’t pull off a complex, 9-11-style attack on the U.S. or Europe. (AP Photo/Hennepin County, Minn. Sheriff's Office)

Screen grab from a Facebook post by Delecia McCain that has photos of her with her brother Douglas McAuthur McCain, also known as Duale ThaslaveofAllah, who was an Islamic State (ISIS) fighter killed in Syria recently, according to an NBC News story. The post also has numerous comments from friends. STORY MUST HAVE CONTEXT OF FACEBOOK PAGE FOR FAIR USE

Screen grab from a Facebook post by Delecia McCain that has photos of her with her brother Douglas McAuthur McCain, also known as Duale ThaslaveofAllah, who was an Islamic State (ISIS) fighter killed in Syria recently, according to an NBC News story. The post also has numerous comments from friends. STORY MUST HAVE CONTEXT OF FACEBOOK PAGE FOR FAIR USE

Sometimes he stayed in a small, one-bedroom apartment with his mother and sister above a Spring Valley hair salon, neighbors said. One observer said he appeared to carry a handgun holstered on his hip.

It’s unclear what McCain did for a living. An American by birth, he smoked and dressed in what another observer described as not the clothes of a devout Muslim.

“Those who are radicalized in favor of jihad often are also marginalized and have had bad experiences with the police and feel alienated from the community,” said Jessica Stern, a professor at Harvard University’s Kennedy School of Government and author of “Terror in the Name of God.”

Money may even be a factor.

“I was surprised when I got to Pakistan how much money played a role in what I always assumed was a career motivated purely by passion,” Stern said.

McCain was killed during a battle in Syria, according to the Free Syrian Army, a rebel militia fighting President Bashar al-Assad’s government. Members of the group told NBC News that McCain was carrying a U.S. passport and $800.

The Syrian Observatory for Human Rights has reported that Islamic State fighters are being paid salaries of $800 a month.

U.S. officials said they have identified nearly a dozen Americans who have traveled to Syria to fight for Islamic State, according to The New York Times on Friday. And the overall influx of male fighters has now prompted Islamic State to begin recruiting foreign women as jihadist wives.

All told, officials said more than 100 Americans have fought alongside rebel groups in Syria since the civil war began there three years ago.

McCain’s mother and sister haven’t responded to phone calls or a message left at their Spring Valley apartment last week.

On social media and in news reports, his family has expressed disbelief that he was fighting with the notoriously brutal Islamic State, sometimes called ISIS or ISIL, whose calling card is broadcast beheadings and death by firing squad.

McCain’s cousin, Kanyata, told NBC that he seemed to gain a strong faith after “he met some Somalian guys who converted him to be a Muslim.”

What’s known about Douglas McAuthur McCain — whose nickname refers to a missing front tooth — is that he was born in Illinois and attended high school outside Minneapolis.

Minnesota, like San Diego, has a large concentration of Somalian refugees.

In high school, McCain reportedly palled around with classmate Troy Kastigar, who went on to fight and die for the al-Shabab terror group in Somalia in 2009. McCain posted photos of Kastigar on his Facebook page, apparently as a posthumous tribute.